Electrography, which broadly includes the forming and developing of electrostatic image patterns either with or without light, has become a major field of technology. It perhaps is best known through the use of electrophotographic office copying machines. Electrophotographic machines and processes have vastly improved since their recent crude origins. Some problems persist, however, and further improvements are needed to extend the usefulness of electrophotography and of electrography in general.
A problem in the development of electrographic charge patterns or latent images has been the difficulty of developing both lines and large solid areas. With most electrophotographic processes the user has had to be content with good quality in one or the other. For instance, copying processes that use cascade development develop lines well, but the solid areas of the image are dense on the edges and faint in the middle--the familiar "fringing" development.
Magnetic brush development as disclosed, for example, by Streich U.S. Pat. No. 3,003,462, improves the balance between line and solid area development. The magnetic brush developer usually is a two-component developer, that is, a mixture of toner particles and of larger carrier particles. The toner is a powdered, fusible resin colored with carbon black or other pigment. The carrier and toner particles have different triboelectric values. As the developer mixture is agitated the particles rub together and the toner and carrier particles acquire opposite electrostatic charges and cling together. In the subsequent development step the somewhat higher opposite charge of the electrostatic latent image draws the colored toner from the carrier and develops the image.
Magnetic brush development uses ferromagnetic carrier particles, usually coated with a resin which aids in triboelectrically charging the toner. A magnet carries the developer mixture of toner and carrier particles and the magnetic field causes the carrier particles to align like the bristles of a brush. As the developer brush contacts the electrostatic latent image, toner particles are drawn away from the carrier particles by the oppositely charged electrostatic image. The copying process is completed by transferring the toned image to paper where it is fused and fixed, for instance, by pressing the paper with a heated roller.
The conductivity of the magnetic brush carrier particles provides the effect of a development electrode positioned close to the photoconductive surface. This aids in the development of solid black areas and of some of the continuous tones in pictures while at the same time providing sufficiently sharp development of lines and dots.
The prior art discloses treatments which improve solid area development by increasing the surface conductivity of magnetic brush carrier particles. The patents to Miller, U.S. Pat. No. 3,632,512 and U.S. Pat. No. 3,718,594, for example, disclose acid treatments which either raise or lower the surface conductivity of iron carrier particles, as desired. The acid-treated iron particles oxidize readily, however, and to remain conductive they must be protected against oxidation. The patent to Miller, U.S. Pat. No. 3,736,257 discloses forming on the particles a thin layer of conductive metal such as nickel or copper by means of electroplating or electroless plating.
These plating methods form stable conductive coatings but have several disadvantages. First, they are costly. Furthermore, although nickel, the preferred plating metal, does not oxidize as readily as iron, it can become oxidized during use, especially if the toner content of the developer mix becomes too low. Then the resistivity of the carrier rises because nickel oxide is an insulator. Also if the carrier particles are not dried well before plating the nickel will oxidize.
Another disadvantage of plating is that the polymeric coating on the carrier, which aids in triboelectrical charging of the toner, does not adhere well to the plating metals. The polymer wears off in use and when it does the toner charge declines.
To solve or reduce these problems, I have developed a novel magnetic carrier component for electrographic developers, a developer mixture containing the novel carrier and a method of preparing the carrier.